The present invention relates generally to receiver electronics in a magnetic resonance imaging (MRI) system, and, more specifically, to an integrated balun-low noise amplifier (LNA) system.
MRI uses radio frequency pulses and magnetic field gradients applied to a subject in a strong homogenous magnetic field to produce viewable images. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it, in random order, at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR receiver coils receive the electromagnetic signals emitted from the patient and use the acquired signals for image reconstruction. Before image reconstruction occurs, the electromagnetic signals received by the receiver coil elements are amplified and filtered to produce an analog signal that can be further processed into an image. One circuit component in conveying the electrical signal from the receiver coil element to the analog conversion portion of the receiver is the balun. Typically, these baluns are constructed as stand-alone components. Such a design can be problematic in that the baluns use large circuit components, that when located in, on, or near the receiver, can interfere with the magnetic flux field and thus reduce the quality and quantity of signals captured by the receiver. Thus, a balun design that minimizes the adverse impact of the balun geometry on the image produced by the MRI system would be beneficial.
Another component in the analog conversion chain of the receiver coil elements is the preamplifier. Similar to the balun, the size of the preamplifier can also interfere with the magnetic flux field and reduce the quality and quantity of signals captured by the receiver. This can be especially problematic as the quantity of receiver coil elements in a phased array increases and the diameter of the coil elements decreases. The decreased diameter of the coil elements results in a higher density of coil elements for a fixed receiver geometry, as there is an increase in the ratio of the electronic component volumetric surface area to receiver coil element area. This increased density of electronic components located near the receiver coil elements effects a higher likelihood of there being an adverse impact on image quality due to the higher relative disturbance of the magnetic flux field.
Therefore, a receiver electronics package that minimizes component area and volume thereof so as to reduce impact on the magnetic field on or near the receiver coil elements is strongly desired.